1. Field of the Invention
The present invention relates to an image processing device. More particularly, the present invention relates to an image processing device for displaying a sub screen within a main screen.
2. Description of the Prior Art
In a conventional image processing device for displaying a sub screen (or "sub window") within a main screen, the sub window having the area either 1/9 or 1/16 of the main screen is selected. In this case, an (1/9) sub window is subsampled at the frequency f.sub.sub /3 and written in a memory, an (1/16) sub window is subsampled at f.sub.sub /4 and written in the memory, and these sub windows are read out at f.sub.main, where, f.sub.sub is a subsampling frequency of the sub window and is 14.318 MHz, and f.sub.main is a subsampling frequency of the main screen and is 14.318 Mhz, the same as the subsampling frequency of the sub window. The frequencies of f.sub.main and f.sub.sub are the same, but they are not synchronized.
FIG. 7 shows a conventional image processing device for displaying a sub window within a main screen. In FIG. 7, a main video signal input terminal 11 is supplied with a main video signal in order to display an image on the main screen, where the main video signal comprises an intensity signal Y and a color signal C. A sub video signal input terminal 12 is supplied with a sub video signal for displaying the image on the sub window, where the sub video signal is a composite video signal. An analog-to-digital converter 14 converts the analog composite video signal input from the sub video signal input terminal to a digital signal. A Y/C separation filter 15 separates the intensity signal Y and the chrominance signal (B-Y and R-Y) from the digital signal produced by the analog-to-digital converter 14, where B indicates a blue signal component and R indicates a red signal component. A color demodulating circuit 16 converts the color signal C into chrominance signals B-Y and R-Y.
The conventional image processing device of FIG. 7 further comprises a window size converting circuit 20, an input terminal 21 for supplying the chrominance signals B-Y and R-Y to the window size converting circuit 20, an output terminal 22 of the window size converting circuit 20, and a data sampling circuit 23 for sampling the respective intensity component Y of the sub video signal and the chrominance components B-Y and R-Y input from the input terminal 21. The data sampling circuit 23 samples the (1/9) sub window by (1/3) f.sub.sub, and compresses the pixels of the respective horizontal scanning lines to 1/3. The data sampling circuit 23 samples the (1/16) sub window by (1/4) f.sub.sub, and compresses the pixels of the respective horizontal scanning lines to 1/4. The compressed image data is read out through a switch 24, and is stored in a memory 25. The image data stored in the memory 25 is read out at the frequency f.sub.main, and the chrominance signals (B-Y and R-Y) of the stored image data are converted into an analog signal by digital-to-analog "D/A" converter 17. Then, the color signal C is mixed with an intensity signal Y in an RGB matrix circuit 18 to form Y and C video signals. The sub video signal output from the matrix circuit 18 is inserted into the main video signal output from the main video signal input terminal 11 by a switch 30. In this manner, the sub window is inserted into the main screen, and the main video signal and the sub video signal inserted into the main video signal are output to the output terminal 13.
FIGS. 8A.about.8D are timing charts of respective signals for inserting a sub window into the main screen. FIG. 8A shows the main video signal input from the main video signal input terminal 11 of FIG. 7. In FIG. 8A, a horizontal synchronization pulse 61 is for horizontal synchronization, a color burst signal 62 is for transmitting information concerning the color signals, and an image signal 63 is for displaying the image on the main screen. FIG. 8B shows a sub video signal 64. The sub video signal 64 has an image signal, as shown by the saw-tooth waveform, which exists only in the portion where the image is displayed on the sub window. FIG. 8C shows a main screen punching signal 65 for inserting the sub window in the main screen. The main screen punching signal 65 is generated by a timing generator (TG) 32. A high speed switch (FSW) 31 controls switching of the switch 30 to the sub video signal side during the period when the main screen punching signal 65 is logic "L", and switching of the switch 30 to the main video signal side during the period when the main screen punching signal 65 is logic "H". FIG. 8D shows a signal during one horizontal scanning line when the main video signal and the sub video signal are superimposed. As FIG. 8D shows, the switch 30 switches to insert the sub video signal 64 in the image signal 63 for displaying the image on the main screen.
FIGS. 9A and 9B show the sub windows which are placed in the main screen. In FIG. 9A, a sub window 81 having 1/16 the area of a main screen 80 is illustrated. In FIG. 9B, a sub window 82 having 1/9 the area of a main screen 80 is illustrated. As explained above, in the conventional image processing device, the sub video signal for sub window having 1/16 the area of the main screen or having 1/9 the area of the main screen have been switched and written in the memory as needed, and the written signal is read out and displayed on the main screen.
However, in an image processing device such as one shown in FIG. 7, it is necessary to use a memory having a large capacity for the memory 25, that is, a memory needed for the 1/9 area of the main screen which can memorize the compressed sub video signal, for example, for one scanning line. When integrating this memory 25 in an IC, it is difficult to reduce the size of the IC, because the size of the IC chip increases in accordance with the size of memory capacity.